Ringer Received: 20 August 2009 / Accepted: 14 October 2009 / Published online: 13 November 2009 Ó to the authors 2009 Abstract Two- and four-probe electrical measurements on individual
Trang 1N A N O E X P R E S S
Electrical Conductivity Studies on Individual Conjugated Polymer
Nanowires: Two-Probe and Four-Probe Results
Yun Ze Long•Jean Luc Duvail•Meng Meng Li •
Changzhi Gu•Zongwen Liu •Simon P Ringer
Received: 20 August 2009 / Accepted: 14 October 2009 / Published online: 13 November 2009
Ó to the authors 2009
Abstract Two- and four-probe electrical measurements
on individual conjugated polymer nanowires with different
diameters ranging from 20 to 190 nm have been performed
to study their conductivity and nanocontact resistance The
two-probe results reveal that all the measured polymer
nanowires with different diameters are semiconducting
However, the four-probe results show that the measured
polymer nanowires with diameters of 190, 95–100, 35–40
and 20–25 nm are lying in the insulating, critical, metallic
and insulting regimes of metal–insulator transition,
respectively The 35–40 nm nanowire displays a metal–
insulator transition at around 35 K In addition, it was
found that the nanocontact resistance is in the magnitude of
104 X at room temperature, which is comparable to the
intrinsic resistance of the nanowires These results
dem-onstrate that four-probe electrical measurement is
neces-sary to explore the intrinsic electronic transport properties
of isolated nanowires, especially in the case of metallic
nanowires, because the metallic nature of the measured
nanowires may be coved by the nanocontact resistance that cannot be excluded by a two-probe technique
Keywords Nanowires Conductivity Nanocontact resistance Conducting polymers Template synthesis
Introduction Recently, one-dimensional nanostructures, such as carbon nanotubes [1], inorganic semiconductor nanowires [2] and conjugated polymer nanowires [3], have become the sub-ject of intense investigations due to their importance for both fundamental research and potential applications in nanoscale devices Among numerous kinds of nanostruc-tures, conducting polymer nanowires and nanotubes, such
as polyaniline, polypyrrole and poly(3,4-ethylenedioxy-thiophene) (PEDOT), are promising materials for fabri-cating polymeric nanodevices By now, electronic transport properties (e.g., electrical conductivity) of nanodevices based on individual conducting polymer nanotubes and nanowires have been explored by various techniques such
as the two-probe technique based on a conductive scanning probe microscope [4 6] The common approach to the two-probe technique is generally realized by dispersing nanotubes/wires on photo- or electron-beam lithographic-prepatterned microleads or nanoleads and the subsequent searching of nanofibers lying on two or four leads only [7 12] In addition, electron- and/or focused ion beam assisted deposition technique has been employed to attach metal microleads on isolated nanotubes/wires [13–18]
A facile technique for fabrication and measurement of polymer nanowire arrays between electrodes in channels was also reported [19]
Y Z Long (&) M M Li
College of Physics Science, Qingdao University,
266071 Qingdao, China
e-mail: yunze.long@163.com
J L Duvail
Institut des Mate´riaux Jean Rouxel, CNRS,
Universite´ de Nantes, 44322 Nantes, France
C Gu
Beijing National Laboratory for Condensed Matter Physics,
Institute of Physics, Chinese Academy of Sciences, 100190
Beijing, China
Z Liu S P Ringer
Australian Key Centre for Microscopy and Microanalysis,
The University of Sydney, Sydney, NSW 2006, Australia
DOI 10.1007/s11671-009-9471-y
Trang 2of the nanowire/tube Up to now, there have been efforts
addressing this problem in the measurements of carbon
nanotubes [20, 21] and individual metal oxide nanowires
such as IrO2 [22], SnO2 [23], ZnO [24] and RuO2 [25]
nanowires For instance, two- and four-probe electrical
measurements on individual SnO2 nanowires have been
performed to evaluate their conductivity and contact
resistance [23] Lin et al [24] have studied the electronic
transport properties of a single ZnO nanowire and RuO2
nanowire [25] through their contacts with a metal
elec-trode However, the nanocontact between a metal lead and
a polymer nanowire has not been precisely explored yet
In our previous works [15–18], we measured the
elec-trical conductivity of isolated conjugated polymer
nanofi-bers and the contact resistance of two crossed polyaniline
nanotubes In this paper, we focus on two- and four-probe
electrical measurements on individual PEDOT nanowires
with different diameters ranging from 20 to 190 nm It was
found that if the temperature dependence of the nanowire
resistance is weak, the resistance of the nanocontact between
a metal lead and a polymer nanowire can dominate the
low-temperature resistance, and thus overshadow the metallic
behavior of the measured nanowire One such case is
when the nanowire is lying in the metallic regime of metal–
insulator transition So a four-probe electrical measurement
is necessary to reveal the intrinsic electronic transport
properties of individual (metallic) polymer nanofibers
Experimental
The PEDOT nanowires were prepared in templates of
polycarbonate track-etched membranes [18, 26–28] In a
typical synthesis procedure, we used a gold layer
evapo-rated on one side of the membrane as the working
elec-trode, a platinum plate as the counter electrode and a
saturated calomel electrode as the reference The
poly-merization bath consisted of an aqueous solution
contain-ing 0.07 M sodium dodecyl sulfate, 0.1 M LiClO4 and
First, we used a scanning electron microscope to find an appropriately isolated PEDOT nanowire on the wafer Then, two pairs of Pt microleads typically 0.5 lm in width and 0.4 lm in thickness were fabricated by FIB deposition (Dual-Beam 235 FIB System from FEI Company, working voltage of the system is 5 kV for the electron beam and
30 kV for the focused ion beam, respectively, current of the focused ion beam is 1–10 pA), as shown in Fig 1 Finally, electrical connection between the Pt microleads and the sample holder was made by highly conductive silver paste and gold wires Electrical measurements of individual PE-DOT nanowires were carried out using a Keithley 236 source measure unit in a helium gas flow cryostat (Oxford),
or a Physical Property Measurement System from Quantum Design and a Keithley 6487 picoammeter/voltage source covering a wide temperature range of 10–300 K The four-probe resistance was measured by applying a very small current (I = 0.01–10 nA, corresponding voltage V = 0.0005–0.02 V) in a range where the I–V characteristics were linear The two-probe resistance was determined under Vbias= 0.02 V The same PEDOT nanowire was used for four-probe measurement first and then for two-probe measurement The resistance of the polymer nano-wire with a given diameter was measured at least twice, for example, under cooling and during heating The reproduc-ibility of the results has been good In addition, for nano-wires with a given diameter, two or more individual nanowires were measured to check the reproducibility
Results and Discussion
As we know, in the four-probe method, the measured resistance R4P is the intrinsic nanowire resistance of the measured segment However, in the two-probe method, the measured resistance R2P is given by R2P= Rlead1 ?
Rcon1? R4P? Rcon2? Rlead2= Rlead? Rcon? R4P, where
Rlead= Rlead1? Rlead2 is the resistance of the two micro-leads and Rcon= Rcon1? Rcon2is the contact resistance of
Trang 3the two microlead–nanowire contacts The two major
fac-tors that affect the contact resistance are the geometry and
the insulating layers (potential barriers) between the
con-tacting surfaces The resistance of a contact is inversely
proportional to its area, and it is dependent on the force
holding the two surfaces together, their stiffness and the
respective electronic structure of the two materials In the
present case, the platinum microlead fabricated by FIB
deposition can promise a good contact with the nanowire
However, insulating layers (potential barriers) between the
nanowire and the platinum microleads are inevitable
because they have different energy levels or work
func-tions In addition, contamination of the nanowire surfaces
from solvent or water adsorption may also increase the
potential barrier width and height
In this study, the resistance Rlead of the two Pt
micro-leads is less than 1 kX (estimated using the widely
rec-ognized resistivity of 5 9 10-4X cm for the deposited Pt
film under the conditions used for the FIB deposition [29]),
whereas the nanowire resistance R4P and the contact
resistance Rconare usually larger than 20 kX (as described
below) So, Rlead is negligibly small compared with R4P
and Rcon, and hence can be ignored, thus we get R2P=
Rcon? R4P It is obvious that if Rcon R4P, then
R2P& Rcon, and if Rcon R4P, then it is R2P& R4P
Through electrical measurements on many isolated
PEDOT nanowires with different diameters, the room
temperature conductivities of the nanowires with diameters
of 190, 95–100, 35–40 and 20–25 nm were obtained that
are about 11.2, 30–50, 490–530 and 390–450 S/cm,
respectively The room temperature conductivity increases
with the decrease of outer diameter of the conducting
polymer nanofibers This was also reported by Martin et al
[30] previously, and could be ascribed to the enhancement
of molecular and super-molecular ordering (alignment of
the polymer chains)
Figure2shows the four-probe and two-probe test results
of resistances for isolated PEDOT nanowires with different diameters ranging from 20 to 190 nm For the 190 nm PEDOT nanowire that is lying in the insulating regime of the metal–insulator transition, as shown in Fig.2a, the two-probe resistance R2Pis quite close to the four-probe resis-tance R4P from 20 to 300 K, and both R2P and R4P have strong temperature dependence These results indicate that compared with the intrinsic nanowire resistance of the measured segment, the microlead–nanowire contact resis-tance is small and negligible For four-probe resisresis-tance of the 95–100 nm PEDOT nanowire, as shown in Fig.2b, it has a relatively weak temperature dependence and is close
to the critical regime of metal–insulator transition It is interesting to find that the two-probe resistance R2Pis quite close to the four-probe resistance R4P at higher tempera-ture; however, at low temperature, R2P increases sharply especially below 25 K and becomes much larger than R4P For the 35–40 nm PEDOT nanowire, as shown in Fig.2c, the result of the four-probe resistance R4P(T) indicates that the nanowire is lying in the metallic regime of metal– insulator transition and there is a transition at around 35 K
It should be mentioned here that R4P-1(T) was measured first, and R4P-2(T) was measured 6 months later However, the two-probe resistance R2Ponly increases monotonously with temperature lowering, indicating that R2P is domi-nated by the contact resistance that is R2P& Rcon, espe-cially below 100 K For the 20–25 nm PEDOT nanowire, which is also lying in an insulating regime, as shown in Fig.2d, both R2Pand R4Phave strong temperature depen-dence It seems that both Rconand R4Pare very large and cannot be ignored for the measured 20–25 nm nanowire Here, it should be noted that although the 20–25 nm PEDOT nanowire has a relatively high conductivity at room temperature (390–450 cm/S), the nanowire shows very strong temperature dependence (R(10 K)/R(300 K) * 105)
Fig 1 SEM images of template-synthesized PEDOT nanowires and the attached four Pt microleads
Trang 4or insulating behavior possibly due to confining effect
limited by the small diameter of the nanowire It is well
known that such an effect should occur when a
character-istic physical length is comparable to the diameter In the
present case, the diameter (20–25 nm) of the PEDOT
nanowire is equal or close to the localization length of
electrons Lc (Lc * 20 nm for conducting polymers close to
the metal–insulator transition [31]); therefore, localization
of electrons induced by Coulomb interaction or small
dis-order must be taken into account in dis-order to explain the
insulating behavior especially at low temperature
By employing the two-probe and four-probe methods,
the electronic contact resistances, Rcon(T), have been
determined We found that the room temperature Rconand
R4P for the PEDOT nanowires are at the same order of
magnitude For example, Rconis 63 and 46 kX, and R4Pis
53 and 24 kX for the measured 35–40 and 20–25 nm
PE-DOT nanowires, respectively However, Rcon(T) increases
rapidly with decreasing temperature, as shown in Fig.3,
indicating an insulating or semiconducting contact formed
at the interfaces between the Pt microlead and the polymer
nanowire Lin et al [25] reported the electronic contact
resistances formed between electron-beam
lithographic-patterned submicron Cr/Au electrodes and single metallic
RuO2, IrO2and Sn-doped In2O3-xnanowires They found
that the contact resistances can range from several tens/
hundreds of Ohm to several tens of kOhm at 300 K, and
their temperature dependences can be well described by a thermal fluctuation-induced tunneling (FIT) conduction model proposed by Sheng [32], which describes the tem-perature-dependent resistance across a single small junc-tion as R(T) = R0exp[T1/(T0? T)], where R0 is a parameter that weakly depends on temperature only, and T1 and T0are characteristic temperatures In the present case, the fitting values for the three parameters R0, T1and T0are
52 kX, 63.6 K and 5.16 K for the 35–40 nm nanowire, and
100k
4P-1 4P-2 2P
10
PEDOT nanowire
T (K)
1M
10k 100k 10M
4P 2P
100
PEDOT nanowire
T (K)
Fig 2 Temperature dependence of four-probe (4P) and two-probe (2P) resistances for individual template-synthesized PEDOT nanowires with diameters a 190 nm, b 95–100 nm, c 35–40 nm and d 20–25 nm
100k 1M
a
30
a: 35-40 nm PEDOT nanowire b: 20-25 nm PEDOT nanowire
T (K)
Fig 3 Temperature dependence of nanocontact resistance (Rcon=
R2P– R4P) determined from Fig 2 c, d
Trang 537 kX, 255.2 K and 4.04 K for the 20–25 nm PEDOT
nanowire, respectively Here, for comparison, the contact
resistances of the 190 and 95–100 nm nanowires have been
the calculated contact resistances that are equal to 11 and
18 KX at room temperature, respectively, and are smaller
than that in the case of 35–40 and 20–25 nm nanowires (63
and 46 kX) It seems that owing to the decrease of the
contact area between the nanowire and the platinum
mi-croleads, the nanocontact resistance at low temperature
increases with diameter decreasing and shows much
stronger temperature dependence
The earlier results demonstrate that the nanocontact
resistance is an important issue in electrical resistance
measurements on isolated nanowires, which may dominate
the measured two-probe resistance especially at low
tem-peratures Compared with the two-probe method, we
believe that the four-probe measurement can further reveal
the intrinsic electronic transport properties of the nanowires
For example, the two-probe results in Fig.2 just indicate
that all the measured PEDOT nanowires are
semiconduct-ing However, the four-probe results reveal the metallic
behavior of the 35–40 nm PEDOT nanowire below 35 K In
addition, for individual RuO2 nanowires [25], it was also
reported that the temperature dependence of two-probe
resistance indicates that the nanowire is semiconducting,
whereas the four-probe resistance dependence of the same
nanowire shows the measured nanowire is metallic
Though the metallic behavior and metal–insulator
tran-sition have been observed in bulk films of doped
polyacet-ylene, polypyrrole, PEDOT, poly(p-phenylenevinylene)
(PPV) and polyaniline [31,33–35], similar metallic
behav-ior and metal–insulator transition have rarely been reported
for isolated polymer nanowires/tubes It is generally
believed that nanosize effect, disorder-induced localization
of the charge carriers and enhanced electron–electron
interaction-induced localization could be possible reasons to
degrade the metallic behavior of nanowires/tubes [3,9,15,
18,36] Based on our results, we propose that nanocontact
resistance may be one of the key reasons for this
degrada-tion In most published results, the temperature-dependent
resistance of a single nanowire/tube was determined by
two-probe technique; therefore, the metallic nature of the
measured polymer fibers could be overshadowed by the
nanocontact resistance especially at low temperatures (such
as the 35–40 nm PEDOT nanowire as shown in Fig.2c)
although the nanofibers show a relatively high electrical
conductivity at room temperature
Conclusions
In summary, we have performed two- and four-probe
electrical measurements on individual conducting polymer
PEDOT nanowires with different diameters ranging from
20 to 190 nm The four-probe results reveal that the mea-sured PEDOT nanowires with diameters of 190, 95–100, 35–40 and 20–25 nm are lying in the insulating, critical, metallic and insulting regimes of metal–insulator transi-tion, respectively The two-probe results, however, reveal that all the measured PEDOT nanowires are semicon-ducting due to the microlead–nanowire contact resistances that show semiconducting or insulating behavior at low temperatures These results indicate that four-probe elec-trical measurement is necessary to explore the intrinsic electronic transport properties of individual nanowires, especially in the case of metallic nanowires due to the effect of the nanocontact resistance that cannot be excluded
in the two-probe measurement
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No 10604038) and the Program for New Century Excellent Talents in University of China (Grant No NCET-07-0472) and by the Communaute´ urbaine
de Nantes, France.
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